Formulating total mixed rations from forages - defining the digestion kinetics of contrasting species.

The work described here is the first step in formulating forage-based TMR rations for dairy cows, based on digestion kinetics derived from in vitro and in sacco incubations. Kinetic information has been obtained from 23 contrasting fresh and conserved forages species, including temperate and tropical grass species, legumes (including Lotus spp. containing condensed tannins), herbs, silages, lucerne hay and maize grain. All forages were minced to a particle size similar to chewed material. Estimates of DM solubility (% DM) and fractional dry matter degradation rates (h ) determined by in sacco incubations include: white clover (39%, 0.195), perennial ryegrass (45%, 0.114), tall fescue (46.8%, 0.051), paspalum (27%, 0.068), sulla (51.4%, 0.121), chicory (41.8%, 0.260), pasture silage (41%, 0.083), maize silage (28%, 0.042). In vitro incubations resulted in five-fold differences between forages in net ammonia release, 2-3 fold differences in volatile fatty acid yield and a three-fold difference in acetate:propionate ratio. The data will be used in simulation models to identify optimal forage mixtures for meeting the nutritional needs of high-producing dairy cows.

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